1. Field of the Invention
This invention relates to superconducting ceramic particles. More particularly, it pertains to a method of preparing small particles of high-purity superconducting oxides, such as YBa.sub.2 Cu.sub.3 O.sub.(7-d) (d=0 to 1), by a freeze-drying technique, which involves atomizing an aqueous solution of the soluble salts onto liquid nitrogen, followed by subliming of the frozen water, calcining and heating in air or oxygen.
2. Description of Related Art
Most reports on the preparation of superconducting ceramic oxides indicate that only crude methods of powder preparation are used, such as calcining the oxide followed by mechanical grinding. The powders produced by these methods are not ideal according to conventional practice. Major problems associated with these powders include inhomogeneity, non-uniformity in terms of particle size, shape, high impurity levels e.g., from milling, and lack of reproducibility. Conducting materials fabricated from these powders may exhibit some superconducting behavior; however, the current densities are usually low and the microstructure is far from controllable. Anomalies in superconducting behavior are often traced to the powder and its size.
An overview of the recent progress in the preparation of superconducting ceramic oxides by Ron Dagani is found in Chemical and Engineering News, pp 7-16, published May 11, 1987, which is incorporated herein by reference. The new materials are metal oxide ceramics, usually having a perovskite-like or spinel-like structure, which can conduct electricity with virtually no resistance at temperatures at or above the boiling point of liquid nitrogen (77K or -196.degree. C.). These materials are useful to conduct electricity for hundreds of miles with no dissipative losses, and no heating up of the transmission lines. These superconductors would also be useful in supercomputers, in magnetically levitated high speed trains, improved nuclear magnetic resonance scanners, and the like.
F. R. Monforte et al., in U.S. Pat. No. 3,516,935 disclose a method of freeze-drying an aqueous solution of soluble salts. It discloses methods of forming the solution, droplet formation, freezing, collection of the frozen droplets, sublimation, conversion, forming and firing. By following the methods described, one would not obtain superconducting powders.
F. R. Monforte et al., in U.S. Pat. No. 3,551,533 disclose a preparation of particulate matter by freeze-drying an atomized solution of soluble salts. A number of solute, solution, freezing, collection, sublimation, conversion, and crushing conditions are described. Sizes of particles range from 1 micron to 0.4 millimeters. This patent does not disclose the process by which superconducting oxide powders are obtained.
A. Lansberg in U.S. Pat. No. 3,357,819 discloses a process of preparing homogenous powders composed of ultrafine particles. A solution or dispersion of the salts are freeze dried by dripping into a cold solution, e.g. liquid nitrogen, followed by sublimation of the water from the particles. The patent does not refer to any subsequent treatment of the particles which is necessary to obtain superconducting oxides.
A. W. Sleight in U.S. Pat. No. 3,932,315 discloses superconductive barium-lead-bismuth oxides of the formula Ba.sub.1-x A.sub.x Pb.sub.1-y Bi.sub.y O.sub.3 wherein A is sodium, potassium, rubidium, cesium, strontium or lead, x is 0 to about 0.5 and y is about 0.05-0.3. The temperature marking the onset of superconductivity is low, a maximum of 13K.
J. Kelly et al., in the Journal of Materials Science, Vol. 13, pp. 1053-1060, published in 1978, disclose a study of a cryochemical method for the preparation of high surface area perovskite semiconducting powders. An aqueous solution is rapidly frozen in liquid nitrogen, followed by a vacuum sublimation of the ice. The sequence of steps necessary to obtain particles of a superconducting oxide of the present invention is not disclosed.
D. W. Johnson et al., in the Ceramic Bulletin, Vol. 53, No. 2, pp. 163-167, published in 1974, disclose the effect of preparative techniques and calcination temperatures on some lithium ferrites. The properties of the particles which were (1) sprayed dried, (2) freeze-dried or (3) co-precipitated were compared. This reference does not disclose the preparation of superconducting powders.
A. C. C. Tseung et al., in the Journal of Materials Science, Vol 5, pp. 604-610, published in 1970, disclose the preparation of high surface area lithium doped nickel oxide particles by spraying an aqueous solution of the salts into liquid nitrogen followed by sublimation and heating at 300.degree. C.-1000.degree. C. to produce the lithium nickel oxide. When silver chloride is present, a large excess of ammonia, to pH 2 or lower, is used to solubilize the silver ion. The materials nor the conditions described disclose or suggest a method of obtaining particles of a super conducting oxide.
V. V. Mirkovich et al., in Ceramic Bulletin, Vol. 49, (#8), pp 724-725, published in 1970, disclose the cryochemical method of preparing ceramic raw materials such as Al.sub.2 (SO.sub.4).sub.3.MgSO.sub.4 by spraying into stirred liquid nitrogen. The particles obtained have a spherical form which varies in size between 50 and 500 micrometers.
P. D. S. St. Pierre et al. in U.S. Pat. No. 3,026,177 disclose a method for producing particles of transparent polycrystalline high density alumina. This patent does not disclose a preparation of superconducting oxides.
Y. S. Kim et al., in the Ceramic Bulletin, Vol. 50 (#6), pp. 532-535, published in 1971 disclose a cryochemical preparation of powders of polycrystalline alumina.
H. A. Sauer et al., in the American Institute of Chemical Engineering Journal, Vol. 18 (#2), pp 435-437, published in 1972, disclose a cryochemical process to prepare particles of aqueous solutions. The droplets are introduced into the lower region of the cooled stirred liquid and rise up through an immiscible nonflamable refridgerant.
S. R. Ovshinsky et al., in Physical Review Letters, Vol 58 (#24), pp 2579-2081 disclose a superconducting oxide of Y.sub.1 Ba.sub.2 Cu.sub.3 F.sub.x O.sub.y having a Tc at 155K. However, other researchers have thus far been unable to repeat this result.
T. H. Geballe et al., in "Some Thoughts About the High Tc Perovskite Superconductors", in the Extended Abstracts for the MRS Symposium in High Temperature Superconductors, Anaheim, Cal., Apr. 23-24, 1987, disclose some physical properties of YBa.sub.2 Cu.sub.3 O.sub.(7-d), where d is 0-1. The bulk sample was reported only as being prepared by a freeze-drying method. No additional description of the experimental details of the method was disclosed.
Other general methods of forming ceramic particles, alloys and bodies are described in U.S. Pat. No. 3,026,210; 3,748,728; 4,073,666; 4,264,358; 4,508,752; and 4,581,289.
It is usually observed that as the size of the particles of the superconducting compounds get larger, that superconducting properties become smaller. Powders having a size of 100 microns or larger (e.g. 200 microns) have reduced or vanishing small superconducting properties. These are the general sizes of the superconducting powders produced by methods of the art described above. Powder having a size of about 10 microns or less show useful superconducting properties. This is the size of the powders obtained in the present invention.
All of the above 20 references are incorporated herein by reference for general information in this art.
None of the references cited hereinabove separately or in combination disclose, teach or suggest the method of producing a superconducting oxide as is described by the present invention.
It is therefore an object of the invention to provide a method of producing a superconducting oxide by mixing soluble aqueous salts to form a solution, atomize this aqueous solution onto liquid nitrogen, remove the liquid nitrogen, remove the ice present by sublimation under reduced pressure, calcine the solid crystals and then heat at 800.degree.-890.degree. C. and cool to ambient temperature to obtain a superconducting oxide having a Tc of 77K or higher and a size of between about 0.05 and 10 microns. The present invention provides such an improved process.